Radiation therapy is used to treat cancers and other conditions in patients. One commonly used form of radiation therapy is external beam radiation therapy. In external beam radiation therapy, a high-energy, x-ray beam generated by a machine, usually a linear accelerator (linac), which is located outside of the patient's body, is directed at a tumor or cancerous cells (i.e., the “target”) inside the patient's body. While the radiation kills the cancerous cells, it also harms normal tissue and organs in the vicinity of the tumor/cancerous cells in the patient. Thus, the goal in radiation therapy is to deliver the required dose of radiation to the target volume, while minimizing the radiation dose to surrounding normal tissue that may cause complications and harm to the patient.
When delivering radiation treatment to a target, geometric accuracy of the delivery of radiation is the key to irradiating the target and minimizing the collateral damage to surrounding normal tissues and structures. It is difficult, however, to achieve the necessary geometric accuracy when the target is moving as the patient is breathing.
The simplest method for dealing with breathing-induced target motion is to have the radiation beam wide enough to cover all possible regions to which the target can move. In the art of radiation therapy, this is referred to as the “addition of treatment margin”. The expanded target is called the “internal target volume” (ITV). This approach, however, results in the inclusion of a significant amount of normal tissues to the treatment volume, leading to an increase in collateral damage and greater toxicity (Coolens et al., Phys. Med. Biol. 53: 4317-4330 (2008)).
Another method that has been attempted to reduce the error due to target motion is to have the patient hold his breath when the target is being irradiated (Korreman et al., Radiother. Oncol. 76: 311-318 (2005)). Since the target is only irradiated when the patient is holding his breath, more time is required to administer treatment because the delivery of radiation is limited to when the patient is holding his breath and the length of time for which the breath can be held. Furthermore, if the desired time at which the breath should be held for optimal irradiation of the target is at the end of exhale, the patient may not be able to hold his breath accurately. In addition, it can be very difficult, if not impossible, to expect a patient with compromised lung function, such as a patient with lung cancer, to hold his breath at all, let alone for a length of time sufficient to deliver radiation to a target.
Another commonly used method that has been utilized to reduce error due to target motion is “gating”. In this method, the radiation beam aperture is reduced to focus on a discrete, pre-selected region of the target at a particular time window during the patient's breathing cycle (Kubo et al., Phys. Med. Biol. 41: 83-91 (1996)). It is assumed that the target motion correlates with the breathing cycle, such that the tumor will return to the treatment position at the same time window of the breathing cycle, during the entire course of therapy. A sensor monitors a patient's breath or abdominal excursion (during breathing) and triggers the delivery of a pulse of radiation at the pre-selected time window. The time window may be selected when the patient's lungs are nearly full as the patient inhales (or, alternatively, when the patient's lungs are nearly empty as the patient exhales). Since radiation is delivered during only a portion (typically 30-40%) of a patient's breathing cycle, the technique is time-consuming and, therefore, not optimal. The duty cycles of such systems are low. In addition, the tumor also moves within the time window, although less than its full range of motion. Therefore, a residual margin is still required to ensure that the tumor gets the intended dose of radiation.
Yet another method that has been explored for reducing error due to respiratory motion is “tumor tracking”. In target tracking, the radiation beam follows the respiratory motion of the tumor. At least four different tracking techniques are currently being implemented or studied.
The first tracking technique involves moving the entire treatment head to track the tumor motion. In existing commercial radiotherapy devices a robotic arm is used to carry the linac around the patient to track the target during the radiation beam delivery. This technique can dynamically adjust to changes in patient breathing patterns. However, it is impossible for non-robotic, commercially available linear accelerator systems to emulate such a method. Notably, only a small fraction of all linear accelerators or external beam radiation equipment has such robotic capability, and therefore, its widespread use is expected to be limited.
The second tracking technique employs static radiotherapy units, which track the tumor with guided breathing (Wong et al., Int. J. Radiat. Oncol. Biol. Phys. 44: 911-919 (1999)). Implementation requires a patient to follow a fixed breathing pattern to match motion of a radiation beam that is pre-programmed according to the guided breathing pattern. This scheme has problems in that most patients are incapable of breathing without irregularities, even after extensive training and with the assistance of audio or video breathing guidance.
A third variant of tumor tracking, which uses a static linac, tracks target motion in real-time and compensates by immediately moving the multi-leaf collimator (MLC) (Keall et al., Phys. Med. Biol. 46: 1-10 (2001)) or the treatment table (D'Souza et al., Phys. Med. Biol. 50: 4021-33 (2005)) to a new, unplanned position in accordance with the detected respiratory target motion. Such an approach requires a quick-responding MLC or table mechanism that would be only possible by the addition of specialized equipment, which is unavailable commercially. Furthermore, such techniques require real-time treatment dose computation and treatment quality verification. Even if tumor position is readily ascertainable, re-definition of live tumor shape changes is necessary and expected to be very difficult, thereby further complicating MLC shape re-configuration. As a result, controlling the movement of the beam to track the tumor is difficult to implement in real-time. Furthermore, this technique also presents a safety issue as the chance for error in calculations performed in real-time increases.
The fourth and more practical method of tumor tracking is described by Yi et al. in U.S. Pat. No. 7,822,716 B2, namely “dose-rate regulated tracking” (DRRT) (see, also, Yi et al., Med. Phys. 35: 3955-3962 (2008)). In DRRT a radiation beam is delivered to a moving target using a pre-programmed MLC sequence designed to track an expected, regular tumor motion trajectory based on tumor motion and breathing pattern data obtained from a four-dimensional (4-D) computed tomography (CT) scan of a patient in need of radiation therapy. Irregularities in the frequency of patient breathing are detected in real-time during treatment delivery, and the information is used to adjust the linac's dose rate to speed-up or slow-down the radiation delivery. Although it handles variations in breathing frequency very well, DRRT does not handle variations in breathing amplitude very well, especially for amplitudes not planned based on regular breathing, such as when the patient occasionally inhales or exhales deeply.
Except for methods that resort to brute force, all previous methods for handling breathing-induced tumor motions are bounded by how the linear accelerator treatment delivery is controlled. The treatment delivery is conventionally controlled by a set of treatment parameters including dynamic beam delivery components such as, but not limited to, each leaf position of the MLC, the MLC carriage, the field size (or collimator opening), the gantry angle, the table position, and the collimator angle, etc. All planned motions of the MLC, the collimator jaws, and the gantry and other parameters are enslaved to the delivered monitor units (MUs), i.e., the treatment delivery machine's internal unit for tracking the amount of radiation sent through its monitoring ion chamber. The relationship between MU and dose in the medium is calibrated by the user.
Thus, the above methods suffer from various disadvantages, such as a need for large robotic linac translation hardware, challenges in determining new beam locations, an MLC design that can adjust rapidly to a new target position in real-time, new hardware to shift a treatment table during beam delivery, and/or a need for a patient to follow a strict breathing sequence, which cannot be achieved even with training and guidance. While these issues are addressed to a significant extent by the DRRT method of Yi et al. by ensuring that the radiation beam of a radiotherapy device is continuously (or nearly continuously) directed at the target, DRRT requires the treatment to follow a preprogrammed treatment sequence that is a function of the delivered MUs. The preprogrammed beam motion sequence is derived by assuming that the patient breathes in a regular pattern while being irradiated, and deviations in the patient's breathing period (i.e., speed) are handled by speeding up or slowing down delivery with dose rate control. Therefore, DRRT requires the linac to vary dose rate in real-time—a capability not readily available for all linear accelerator designs. Moreover, because dose rate control can only vary the speed of the delivery, DRRT does not enable handling deviations in the patient's breathing amplitude. While it is theoretically easier to adjust dose rate when the electron gun is “gated”, it is difficult to make quick and accurate adjustments when more efficient “non-gated” electron guns are used as the source of electrons being accelerated.
In view of the foregoing, it is an object of the present disclosure to provide methods and a system for target tracking that do not suffer from the disadvantages attendant the methods of the prior art. This and other objects and advantages, as well as inventive features, will become apparent from the detailed description provided herein.